Voltmeter



United States Patent() VOLTMETER Howard W. Ecker, Reynolds Marchant, and Raymond J. Wagner, St. Paul, Minn.

Application October 26, 1955, Serial No. 542,975 7 Claims. (Cl. 324-72) r1`his invention relates to voltmeters, and is particularly concerned with meters for measuring high potentials on surfaces of low conductivity and low capacity.

Electrostatic potentials of considerable magnitude are frequently generated on power transmission belts, continuous moving webs of paper or plastics, and other objects of low conductivity. These high potentials have caused explosions and tires by producing spark discharges in the presence of explosive vapors and combustible materials. Spark discharges of otherwise harmless intensity have been the cause of accidents to unwarned and unsuspecting personnel. Generation of high potentials on traveling webs of paper has resulted in offsetting of ink and other coating or printing materials, necessitating frequent stops for cleaning of apparatus, and producing printed or coated products of inferior quality. These and other harmful effects may often be eliminated by proper grounding or neutralizing of the charge; but eiective control and elimination of these undesirable high potentials is dependent on their detection and measurement.

In other instances, measurement of high potentials may be desired in order to maintain a particular system within required voltage limits, or to determine rapid changes in voltage, or for various other purposes. A

Electrostatic voltmeters, which operate on the forces set up between adjacent charged plates or vanes, are capable of measuring high voltages; and attempts have been made to use such instruments for the type of measurements just indicated. These voltmeters require a high charging current and exhibit high energy storage. They are usually bulky and delicate, and must be leveled before use and protected from mechanical shock.

Another method for measuring high electrostatic potential involves the use of a capacity divider in combination with a low voltage, low current indicator. The probe or test electrode may serve as a portion of the divider circuit, in which event the spacing between probe and test surface must be carefully controlled. Capacity dividers cannot be used for continuous measurement of variable voltage but require discharge of the system after each determination.

The present invention avoids these and other defects and deciencies of prior art high-voltage measuring systems. It provides means for measuring high voltages, particularly above a few hundred volts, with surprising accuracy and substantially independently of probe position. Both positive and negative voltages are measured and identied. Danger ofharmful sparking between source and instrument is eliminated. Thrue voltage readings are obtained from both low conductivity and high conductivity surfaces. Measurements are sufficiently rapid to permit continuous determination of voltage on traveling webs of paper or the like. Furthermore the instrument may be made easily portable if desired, or may be permanently installed, e.g., as a recording or controlling voltmeter unit.

These and other advantages are obtained by employing a current-flow type of indicator instrument in combination with a novel probe element capable of rapidly establishing equilibrium with the test surface voltage regardless, at least within wide limits, of the geometric relationship existing between the two. The current-How indicator is rugged and portable. The probe unit cooperates with the indicator to provide the novel results described.

The principles of the inventionr will now be described in connection with the accompanying drawing, in which Figure l is a circuit diagram, in conventional form, and Figure 2 is a view in perspective, of a portable voltagemeasuring unit embodying these principles.

The exemplary measuring unit illustrated in the drawing includes a probe unit 10 and a meter 20, the two being interconnected by means of a suitable shielded cable 30, in this case a flexible cable since the apparatus is designed for portability.v The shield 31 grounds the metal hand grip 15.

The meter unit 20 includes a microammeter 25 in the output circuit of the triode 24 as illustrated. The test current from the probe unit 10 induces a potential across the grid of the triode. The range selector switch 21 provides for holding the indicated voltage within the scale of the meter 25. Zero setting of the meter is obtained through adjustment of potentiometer 28 prior to making a measurement. Calibration of the instrument is accomplished by adjusting potentiometer 26 while activating the probe with a test surface at a known voltage.

In a typical meter circuit of the type illustrated the values of the several resistor components are as follows:V

R1 megnhms 500 R2 do 5,000 R3 Ohms 75,000 d0 30,000 R5 do 30,000 d0 100,000 do- 100,000

The triode is a low grid current type electronic amplilier. The plate voltage is 30 volts.

The surprising and advantageous results obtainable with the voltmeter of this invention are due in large part to the combination of the sensitive but rugged electronic current-'flow voltmeter unit with the novel directional probe unit now to be' further described. As illustrated, the vprobe unit 10 comprises a hollow frusto-conical tip 12 terminally mounted on a protective insulating handle 14. An electrode 11 is mounted at the base of the conical tip. For use with the specific meter unit described above, the conical tip is three-fourths of an inch inside diameter at the open end, and the vertical distance from the surface of the electrode 11 tothe open end of the cone is also three-fourths of an inch. The angle formed by opposing side elements of the cone is 20 degrees. The cone, and the tubular handle to which it is attached, are formed of polystyrene or other equally effective moisture-resistant insulating material.

vThe tubular handle 14 contains a small protective resistor 16 directly adjacent the electrode 11 and a series of high resistance current-limiting resistors 13 spaced along at least a major portion of the forward interior of the tube. These several resistors 13 have an extremely high total resistance value, being in the example here given about six million megohms.

The operation of the device is particularly dependent on the nature of the electrode 11, which, for the purposes here set forth, is required to be radioactive `or capable of causing ionization of the air or other gas with which it may be in contact. In the specific embodiment here described, the electrode 11 is in the form of a flat metal disc having a face area of one-tenth square inch and plated on the exposed flat face with a total of micro-.- grams of radium.

'- voltage range to be tested.

'The meter is first calibrated by reference to a test surface at a known potential, for example, a at metal panel connected to a high-voltage battery. The openend of the yconical tip of .theprobe .is held facing the panel and .at :a .distance of about l-.3 inches, while the potentiometer .-26 fis adjusted by means ofscrevv 26 :to the approximate 'corresponding scale reading. 'I he probe lis then Yremoved from the source Yof potential .and the zero setting ofthe meter .is re-adjusted by :means .of potentiometer 28, .operated 'through .the knob 28. One .or more subsequent .re-,checks against .the `known potential and IIe-.adjustments `of the zero setting .may he .necessary if a high degree lof Yaccuracy is desired. Once the .calibration is completed, any slight drift of the vZero reading during use of storagemayfbe compensated by. adjustment of potentiometer 428 'without disturbing the calibration,potentiometer l21S; Y Y

Swith 21, -operated through knob ,21 provides meansV for selecting Lthe proper meter scale .corresponding to the `It is to benotedthat the specific distance between-.probe jand test surface, withinthe approximate limits suggested, is not in anywaycritical. Thus the probe maylbe fixedin place, facing a moving web of paper or other-materiahto provide continuous analysis of surface .voltage regardless .I 4 extremely high voltages may be significantly reduced; but lower voltages may be accurately measured. Substantial- Vly increased resistance in the probe unitmay require a may be better appreciated, it is .pointed out that under Optimum .conditions a .current .of `only about 0.0.0.8 .misfov ampere .per 50 killovolts is required. i Operation 'is possiblewith currents as high as 0.1 microampere per 50 i kilovolts but the accuracy -of measurement is substantially reduced when measurement 4of potential existing on of minor flutter or -sag in the web; `a result Whichhas` Y not .been .found possible with previously `available .highvolatge electrostatic .measuring .deviees,..so Sfar .as yweh'ave been :able to determine. The determination isfrapid, and succeeding voltages zat greatly differing .amplitude or .opposite polarity are equally effectively .registeredon the meter. Y Y

The nature of the probe unit, and the method of operation of the device, ordinarily provide Yassurance against accidental spark discharge eitherto ground 'bor Ato 'the operator. As a 'furtherprecautiom'the resistor 1'6,hav ing 'in the "specific circuit illustrated arvalue of 'approxidielectric or insulative surfaces is made with current drains of this magnitude.

While the invention has been described primarily in termsrof a preferred portable unit and with .components of specified constants,vit will .be apparent that `'modifications'can bereadily provided within the skill of the art. Thus :the voltage andcurrentsupply to .theelectronic amplifier may'beobtained from .an alternating-.current source by means of suitable well-known transformer, rectifier andlter means; the results obtained may be drectly recorded by arecording type -voltmeter, .or may be transmitted vto a zsuitable alarm system; and yother Vnon-invel" tive 'modifications vwill be apparent. .In like manner, polonium er other emitter pf alpha .particles .or .of .19W energy :betaA particles may replace Vthe Vradium Von the collector electrode 11, and the area and shape of that electrode, as well as the speciicvform and dimensions o f the tip 12 ,may also be altered. However the specic structure l.hereinbefore describedhas been found .in practice to mately 100 'in'.egohms, 'may Ybe placed uimmediately Aadf energy-which might be lstored `in `the connectorwire `between. the plate and the resistors .13 or .in ,other Tmetal parts. Where the capacity Vof such components isV extremely. small, thisresistor may ordinarily he'omitted. j

The several lresistors 13, totaling six million megohms, are fnunits of a physical size and shapepermitting their enclosure within the tubular insulating handle 14, .which ordinarily is desirably curved, asY illustrated vin Figure.2, for convenience in positioning the tip 12. In a-.specilic instance, three two=million megohm cartridge `type resistors in series, each'about 4iinches long, ihavinga total installed length'of approximately 12 inches, .andxextend- VVjacent the'plate vll'topreventfany spark-discharge of be highly suitable, being welladapted to -the .rapid determination of high voltages withoutrharmful discharge Vand .at variable distances.

Having now'described our invention in terms of s peciiicillustrative embodiments, but without intending'to beglimitedthereto, what we claimis as follows:

d l. Avoltagefmeasuring-deviee suitable for rapidly. and continuously measuring variable high stat-ic voltages without significant alteration of ambient electrostatic .field and without-.dangerof harmful sparkrdischarge, said device comprising Aa directional, externally insulated, conductive, radioactivev probe, a low current input electronic voltm'etefr, and a high resistance connectingisaidprobe and said -voltmeter and uniformly distributed within the ing to a distance of about 15 inches Vfrom the plate 11, Y

were employed. A single extended-length vresistor of proper shape Vis equally elective'but.ordinarily'isnotias readily available. In either case, .thegresistance `is,ex tended .uniformly fromthe tiplZ'overat leastarconsider.-

able .portion of the length of the tubular handle,and.no

Yshield or ground element is in proximity.

The surprising speed and accuracyof operation ob Y tained with ftheicornponents described .isbelieved `vto 1 be dueto the establishing of a..potential;gradient,;along1the series of resistors Y13, which is' roughly .commensurate with the vpotential'gradient established along theicorre-V Y.

spondingly Vlocated outer surface of theinsulating handle 14 .by the presence .ofthe-electrostatic charge which isgto be measured.V In any eventthearrangement has -been found tozprovide ;a probe system v'rhichfrapidly:rea'ches equilibrium :in` an electrical field .with :a minimum p ow of fcurrent .and `:without signicant alteration of ;the ambient field. Accordingly, -the "measurement of .true voltagesY onfpaper, plastic ilms, Yand otherv dielectric or insnlativeand low-capacity surfaces is` made possible.

Thetstotal `resistanceYof-the resistors 13, while .necessarilyfzhi'gh, is not critical. 'At Avalues-.-.substantially .l less than six million megohms the accuracymfrithe meter:.:a t

insulating. l1andle ofsaid probe. v Y Y 2. A voltage measuring .device as defined `in claim l .inwhieh the radiactiveprobe iS loeted .Within a lowf angle, forwardlyxopen, hollow conical insulative shield forcontrolling the direction and `cross-sectionalarea of the ionized path between probe ,andtest surface.

.3. Afvoltageimeasuring device as de iinediin claim l and -capable of continuously measuring variable high static voltages on ystufacesfrorrl which, current flow is restrcteddo not greater than about l0.1 Ymicroampere perV 50 kilovolts, in which the radioactive probe contains a source fof 'radioactivity equivalent ,to-about l5 0 micrograms .etradium and -the high resistance. connection has aresistance. of labout .six million. rnegohms.V

.4. For. use in a highvoltage measuring. device, ajdirectional radioactive probemember comprisingsan-,insulatingshieldhaving a Vlow-angle frustum-shapedvopeninterior, open at the'larger end and containing'at -the smaller interior end a radioactive collecting -electrode,, and conductive 1connector means attached to said electrodeY for making velectricalcontact there-with. Y

V5.1301 Y.use in ahigh voltage, low Vcurrent lmeasuring device A.having -a conductive radioactive probe connected to ,a current-ow .type indicator, connectormeanscornprising a high resistance element V uniformlydistributed along'theforward interior of tubular insulatinghandle fQILSid .ipl'Qbe, -and a g relatively low resistance protective element at the forward end of said handle and connecting said probe and said high resistance element.

6. A directional radioactive probe member consisting essentially of a plastic open cup-shaped insulating shield member having a dat base and outwardly aring truncated conical walls, a flat metallic collecting electrode member over the inner surface of said base member and having a radioactive exposed surface, and a conductive connector element attached to said electrode and passing through said base.

7. A probe and handle unit for a high-voltage voltmeter assembly, comprising an elongate tubular insulating handle member terminating in an open cup-shaped insulating shield member having a at base and outwardly aring truncated conical walls, a at metallic collecting electrode member over the inner surface of said base member and having a radioactive exposed surface, an

elongate resistor element disposed uniformly along the interior of at least the forward portion of said handle member, a conductive connector element passing through said base and connecting said electrode and the forward end of said resistor, and means for conductively connecting the rearward end of said resistor with potentialestimating means.

References Cited in the file of this patent UNITED STATES PATENTS 1,919,215 Gunn July 25, 1933 2,084,968 Chapman June 22, 1937 2,087,915 Kimball July 27, 1937 2,457,575 Liebscher Dec. 28, 1948 2,466,211 Crockett Apr. 5, 1949 

